In well logging activities, a gamma ray measuring device formed of an elongate cylindrical scintillation crystal is normally used to detect gamma radiation. Ordinarily, this measurement is made in a non-directional fashion. That is, the crystal is responsive to gamma ray impingement from all directions of azimuth with respect to the well borehole. In many circumstances, it would be desirable, indeed helpful, to have a gamma ray measurement which is responsive to the direction of impingement. The present apparatus is directed to this, namely, provision of the azimuth of the gamma radiation. In the past, directionally sensitive devices have utilized a type of cylindrical shutter or collimator. Such a device typically has been made of lead or tungsten to provide suitable gamma ray attenuation. The shutter or collimator is typically rotated with an end located drive motor. Depending on the mode of operation, it can be simply rotated or it can be stopped after discrete rotation increments. In whatever fashion it is used, the window or port in the shutter is directed in a particular direction, counts are recorded for a specified interval, and the directional sensitivity is thus provided. As will be understood, this requires rotating a sleeve within a sleeve referring to the shutter which typically is a rotating sleeve mounted within the elongate tubular housing of a sonde. Other methods include clusters of detectors whose efficiencies are relatively low, and the associated electronics to maintain relative balanced sensitivity is complex.
The present apparatus sets forth a method and apparatus for detecting the direction of impingement by first determining the point along the length of the scintillation crystal as which the gamma ray impinges. The apparatus is simple electronically, and has no moving parts. For definitional purposes, assume that photomultiplier tubes are located at both ends of the scintillation crystal. Assume further that both of the PMTs provide output signals A and B indicative of a particular photon impingement. In accordance with the teachings of U.S. Pat. No. 4,618,765, the point X along the length L of the scintillation crystal is given by the relationship X=L[1+(A/B).sup.1/2 ].sup.-1.
Consider the easy case where the output signals A and B from the end located, sensitivity matched, PMTs are exactly equal. In that example where A and B are equal, the denominator of the foregoing equation is 2; in that instance, the variable X is one half of the length L. The referenced patent also provides information regarding the energy of the impinging gamma ray which is a function of the output signals A and B and the physical dimensions of the scintillation crystal.
The present disclosure takes advantage of the foregoing in conjunction with a direction measuring device in the sonde such as a reference gyroscope. A reference gyroscope is preferable because it can be used in uncased and cased wells. Alternately, magnetic measuring devices can also be used so long as there is no casing and the sonde is substantially free of ferrous metal influence. In any event, that provides a reference direction so that the particular azimuthal position of the sonde at the instance that data is obtained can be measured. The present disclosure describes a collimator structure arranged around the scintillation crystal. The collimator is a sleeve of material such as lead or tungsten which readily absorbs gamma radiation. Further, the sleeve is fixed in position with respect to the scintillation crystal. Further, the sleeve has a window cut in it which interrupts the sleeve. The window is a helix of one full revolution from top to bottom matching the length L of the scintillation crystal. As an easy example, if the scintillation crystal is precisely one foot in length, then the helix provides one full turn per foot, so that 360.degree. of azimuthal perspective is obtained.
The present apparatus further includes a fixed reference for some direction and the preferred form is a gyroscope which provides an indication of north. Thus, in the method disclosed hereinbelow, a north reference is established, and at any given instant, when a gamma ray impinges in the scintillation crystal, direction can be determined in a relative fashion, and also in an absolute fashion with respect to the reference. The scintillation event caused by the gamma ray is detected at the two respective end located PMTs, and the position of the event along the length of the scintillation crystal is first determined as taught in U.S. Pat. No. 4,618,765. Once that lengthwise location is known, the azimuthal direction relative to the scintillation crystal is known without ambiguity, and the absolute azimuth of the direction can then be determined.